Joseph slepian



' 1,642,091 p 1927- J. SLEVPIAN .CONSTANT CURRENT DEVICE FOR GLOW DISCHARGE MICROPHONES Filed April 21. 1924 WITNESSES 0 Amperes INVENTOR Patented Sept. 13, 1927.

UNITED STATES JOSEPH SLEPIAN, OF SWISSVALE, ZENNXLYLVAEH ELECTRIC & MANUFACTURING o ASSETS-NOR TO WESTINGHOUSE a, at. tKllCtP-UIMLTEON OF PENNSYLVANIA.

CONSTANT-CURRENT DEVICE FOE G'LOW DISOHARGE MICROPHONES..

Application filed April 21, 1924. Serial No. 767,846.

My invention relates to sound-translating systems of the type in which a glow discharge is the sound-sensitive element.

It is an object of my invention to provide 6 a glow-discharge telephone transmitter sys tem which is adapted to provide a maximum sensitivity of response to sound waves.

It is a further object of my invention to provide means whereby the current in the 10 glow discharge may be held at that value at which there 1s maximum sensitivity coupled with stability.

It is a further obiect of my invention to take advantage of the saturation characteristics of a vacuum-tube device for regulating the current supplied to a microphone of,

the glow-discharge type.

In the application of Phillips Thomas,

Serial No. 600,822, filed November 14%., 1922,. and assigned to the. Westinghouse Electric and Mfg. (30., is described a glow-discharge sound-sensitive device and the various phenomena afiecting the sensitivity of a glow discharge to vibratory influences, such as sound waves.

The glow-discharge microphone is more sensitiveto sound when small currents employed than with larger currents. The glow-discharge has a negative-resistance characteristic. For this reason, it is essentially unstable and can be maintained only by employingsome expedient for automatically regulating the voltage or current; The smaller the current, the greaterthe diiliculty inovercoming this instability. This inven' ;-tion prov-ides means for more eflectually reg ulating the current so thatthe glow discharge may be maintained at currentsmuch smaller than has heretofore been possible 40 and, consequently, can be made to possess much greater sensitiveness to sound than heretofore.

The great effectiveness and reliability of the current-controlling device provided by this invention enables the glow-discharge microphone to be used with currents so small that the stability is limited'by circumstanres which have, heretofore, exercised no appreciableinfiuence thereon. The capacity e'ttects between the electrodes of the glow-discharge give a lower limit to the current by introducing causes of instability not heretofore appreciable. This invention. enables the currentin the microphone to be diminished to the point where these capacity effects render the discharge unstable. This is far below the.

current values heretofore employed.

It has been proposed to limit the current through a glow-discharge microphone by inc ruling a resistance in series therewith. To obtain the desired eifects,this resistance must be made very large which requires that the potential impressed across the resistance and microphone in series must be very large. This very large potential is a source of inconvenience which the present invention overz-omes. y Other objects and structural details of myinventionwill be apparent from the following description when read in connection with the accompanying drawings, wherein:

Fig.1 is a diagrammatic view of apparatus and circuits embodying my invention, and p I 2 is adiagram showing curvesto which reference will be made in the explana tion ot-the operation of this invention.

In Fig. 1,1 have shown a glow-discharge transmitter 1, an amplifier 2, a constantcurrent device 3, a filter device 4, a rectitier device 5 a current supply transformer 6 and a source of alternating current 7. The transformer 6 i s constructed with a primary .conneited to. the alternating-current supply from which ,it is desired to operate the system and a-sec ondary giving .a high voltage to feed to rectifier 5, which may be,

indicated, a full wave thermionic discharge rectifier.

vThe filllelt may comprise any desired combination of inductance and capacityv which will produce a partial smoothing out of the fluctuations in the rectified current supply. It is not necessary that a complete filtering action take place. Furthermore, the .tiltcr 4. may, on occasion, be omitted without serious detriment, but its presence is convenient.

The constant-current device 3 comprises a thermionic discharge device having an anode 11, a cathode 12 and a cathode heating and control means 13. The electrodes of the constant-current device 3 are insulated from ground. its cathode being connected to the negative terminal of the filter i and its anode being connected to the Ill cathode of the glow-discharge transmitter 1.

The electrodes 11 and 12 are mounted within an evacuated container 14, being sealed through the glass walls at opposite ends of the device, thereby providing for maximum current-creepage distance for insulation between the anode and the cathode.

In Fig. 2, the curve a shows a relation between the current and the voltage in the glow discharge. It will be observed that this current slopes downward and to the right, which indicates that the glow-discharge has a negative resistance characteristic. At the left hand end of the curve, this slope is very steep, indicating that the negative characteristic is very large and, consequently, the glow discharge at these values of the current and potential is very unstable. Further to the right the curve is less steep, indicating that it is less difficult to maintain the glow discharge.

It is usual to maintain a constant current through the glow-discharge device by means of a device in series therewith. With the point 0 for an origin, current through the device thus placed in series is exhibited by the curve Z), when the device is a resistance, and by the curve 0, when said device is the vacuum tube described above. For these curves, the voltage is measured downward so that if the distance 0 0 represents the voltage impressed upon the two devices in series, the intersection of the curve I) or c with the curve a will represent the conditions at the microphone.

The curve 0 exhibits the saturation characteristic of the vacuum-tube device 3. After the saturation point is passed, the current through this tube is substantially independent of the voltage impressed thereon. Consequently, the lower part of the curve 0 is substantially a vertical straight line. On the other hand, the curve I), showing the current when the series device is an ohmic resistance, is a straight line of constant slope and the tangent of this slope equals the numerical value of the resistance.

It is found that the stability of the glow discharge is a function of the difi'erence between the tangent of the slope of the curve a and the tangent of the slope of the curve representing the current through the series device. Fig. 2, therefore, makes it obvious that greater stability is obtained with a current-controlling device having a saturation characteristic than with an ohmic resistance. It will be obvious that any other current-carrying device, in which the current. at all voltages above a certain limiting minimum, is independent of the im pressed voltage, will serve as well as the vacuum tube device illustrated. The expression current-carrying device of the saturation type is used in the specification and claims with the intention that it shall include all devices having this characteristic.

Since the sensitiveness of the microphone to sound increases as the glowdischarge becomes more unstabie, it is obvious that the microphone should be worked on as steep a part of the curve a: as possible. If the intersection of the curve a with the other curves is to be farther to the left and the series device has only ohmic resistance, it will be evident that the curve Z) must be made steeper. This can be done by increasing the resistance, but this re( uires that the impressed potential 0 0 be also increased. When this potential is several thousand volts, insulation difiiculties become exceedingly troublesome. Moreover, the line Z) is never vertical, so that the angle between it and the line a is never as large as it would be if the curve representing the current in the series device were vertical.

lVhen a current-limiting device of the saturation type is employed, the point of intersection with the curve a may be moved to the left by lowering the voltage required to produce saturation. This does not require any increase in the impressed voltage 0 0. Insulation difiiculties, therefore, are not increased by a device of this type. Moreover, because the part of the curve representing the current in such a device after the saturation point is reached is vertical, the angle between this curve and the curve a is as large as can be obtained.

l/Vith a vacuum-tube device like that illustrated in Fig. 1, the voltage necessary to produce saturation is regulated by adjustment of the controlling resistance 13. It is, therefore, possible, by adjusting this resistance, to bring the microphone current to such low value that the disturbing effects due to the electrostatic capacity of the electrodes produce inst-ability. This electrostatic capacity, by tending to set up oscillations, causes the glow discharge to become unstable, after the current is diminished beyond a certain limit; but this limit is far below the value of currents heretofore employed with glow-discharge instruments.

While I have shown only one embodiment of 1 y invention in the accompanying drawings, it is capable of various changes and modifications without departing from the spirit thereof, and it is desired, therefore, that only such limitations shall be placed thereon as are imposed by the prior art or set forth in the appended claims.

I claim as my invention:

1. In combination, a glow-discharge transmitter and constant-current means comprising a thermionic discharge device operated at its saturation current, serially connected in circuit with said transmitter.

2. In combination, a source of alternating current, a transformer, a rectifier, a filter, a

3. In combination, a source of alternat-i ing current, a transformer, a rectifier, a filter, a current-limiting device, a glowdischarge sound-responsive device, an amplifier evice and an output circuit, serially connected, said current-limiting device comprising a thermionic device having a heated cathode, an anode, and an evacuated container surrounding said anode and cathode.

4. In combination with a glow-discharge transmitter, constant-current means comprising a thermionic discharge device operated at its saturation current and serially connected in circuit with said transmitter,

said cathode being connected to a source of heating current, and a rheostatic control means.

5. In combination, a glow-discharge device, a current-carrying device of the saturation type in series therewith and means for impressing a direct potential thereon.

6. In combination, a glow-discharge device, a current-carrying device of the saturation type in series therewith, means for adjusting the saturation potential of said current-carrying device and means for impressing a direct potential thereon;

In testimony whereof, I have hereunto subscribed my name this 9th day of April,

JOSEPH SLEPIAN. 

